In a U.S. Pat. No. 3,777,280 issued on Dec. 4, 1973 to Dieter Pohl and assigned to applicants' assignee, the TE.sub.01 mode is favored whereas the other modes are suppressed. This discrimination is achieved by employing in the laser cavity a birefringent crystal, zero degree cut calcite, which deflects the beams in the crystal so as to suppress the TM.sub.01 mode but favor the TE.sub.01 mode. In such Pohl invention, however, there is no teaching of how to switch back and forth in a single laser from the TE.sub.01 to the TM.sub.01 and vice versa.
In yet another teaching (See "Generation of Radially Polarized Optical Beam Mode by Laser Oscillation" by Y. Mushiake et al, Proc. of IEEE, Vol. 60, Sept., 1972, pp. 1107-1109) a He--Ne gas laser was operated in the TM.sub.01 mode having its magnetic field azimuthally polarized and its electric field radially polarized. By passing the TE.sub.01 beam through a rotationally symmetric 90.degree. rotator, the TM.sub.01 mode can be switched to the TE.sub.01 mode and vice versa. However, Mushiake et al's device has the limitation that the amount of rotation produced by the rotationally symmetric 90.degree. rotator is wavelength dependent.
The present invention achieves the capability of switching back and forth from the TE.sub.01 mode to the TM.sub.01 mode so that such switching is independent of the wavelength of the laser. Such wavelength independence is attained by the use of a mode selector which comprises a zero degree calcite crystal inserted between two lenses in the laser cavity. One lens is concave which makes the light strongly divergent as it passes through the calcite. The other lens is convex which recollimates the divering beam into a parallel beam, the latter being reflected from a mirror or diffraction grating of the optical cavity. The divergent light beam undergoes double refraction in the crystal, which allows for discrimination between TE.sub.01 and TM.sub.01 modes. By varying the spacing between the convex and concave lenses, switching between the TE.sub.01 and TM.sub.01 mode is achieved that is independent of the wavelength of the oscillating light in the laser cavity.
It is an object of this invention to provide a novel laser capable of selecting either the rotationally symmetric TE.sub.01 mode or the rotationally symmetric TM.sub.01 mode.
It is yet another object of this invention to provide such selection capabilities in a laser that is independent of the wavelength of the laser.